Above: Equipped with remote sensing technology developed
for the space program, "heat hunters" fly NASA aircraft
over urban areas, documenting patterns of heat formation in large
metropolitan centers. This
simulation shows how these remote sensing studies identify
thermal "hot spots" in metropolitan Atlanta and its
suburbs. It also shows potential weather phenomenon created by
Atlanta's sizeable urban heat island.

Heat islands are created when city growth alters the urban fabric
by substituting manmade asphalt roads and tar roofs and other
features for forest growth. Trees provide shade and cool the
air through evaporation. The hard, dark surfaces like pavement
store heat during the day, heat that is then released at night,
keeping the city hotter for longer periods of time. Air-conditioning
is then kicked into overtime to combat the heat.

Sign up for
our EXPRESS SCIENCE NEWS delivery

The urban heat island phenomenon was first discovered
in the early 1800s in London. The focus of research now is on
the driving forces, magnitude and overall extent of the effect.

Quattrochi says that two major goals of their studies are to
"understand how the characteristics of the urban landscape
drive this urban heat island effect and how urbanization and
growth shape the dynamics of the effect."

One concern is the ability of urban heat islands to create or
alter weather patterns - specifically to cause thunderstorm activity.

As heat is generated and trapped in a city, hot air rises
and forms a "chimney sitting over a city [and] ... sucks
in the cooler air," creating winds. The dome of heated air
generates convective clouds that bring rain and thunderstorms.
Urban heat islands cause precipitation to fall at unusual times
like 2 a.m. - "not your typical garden variety southeastern
storm," says Quattrochi.

The 1996 Olympics in Atlanta afforded scientists a chance to
scrutinize data collected by the National Weather Service's ground-based
meter used to predict weather for the athletes. Urban meteorologists
Robert Bornstein and Qing Lu Lin of San Jose State University
found that 5 of 9 days of precipitation were caused by the urban
heat island effect.

By analyzing Landsat 7 images from the 1970s to the present,
scientists can determine trends in urbanization and rising temperatures.
They found that suburban development in Atlanta gobbled up 380,000
acres of trees from 1973 to 1992.

Right: Heat builds in a city when
forests are cut down to build roads. Not only do paved roads
hold in heat, cars also compound the problem by generating smog
and more heat (inset). Photo credits: Department of Energy, Warren
Gretz.

While much of the growth in Atlanta has been residential in nature,
Quattrochi cautions that commercial and residential development
often go hand-in-hand. As Atlanta expands, more trees are cleared
to accommodate the growth. Trees are replaced with paved roads
that retain heat and host the deluge of traffic -- traffic that
now must travel the longest average daily commute in the nation.

Vehicle emissions and rising temperatures also contribute to
an increase in ozone, a pollutant detrimental to the environment
and human health. During last year's ozone season in Atlanta,
which runs from the end of April to the end of September, the
city suffered through 62 straight days of ozone alerts. Quattrochi
says that, based on models, there is potential for a temperature
decrease of 2 degrees Celsius in Atlanta to lower the ozone by
10 to 14 percent, a significant drop.

With no indication that urban sprawl will slow in the near future
scientists are searching for ways to curb urban heat islands.

Quattrochi and other team members hope that state and local governments
will use their findings as a springboard to adopt "cool
community measures." These alternatives include planting
trees and using reflective roofing materials that can reflect
60-80 percent of incoming sunlight, more politically viable options
than air quality restrictions.

The 1996-2000 NASA-funded study in Atlanta, The Atlanta Land-use
Analysis: Temperature and Air-quality (ATLANTA), has been mirrored
by the Urban Heat Island Pilot Project (UHIPP). This study sequence,
also funded by the US Environmental Protection Agency, has been
conducted in Baton Rouge, Sacramento and Salt Lake City. Up next
for satellite data analysis are Los Angeles, Phoenix, Tucson,
Chicago, Nashville and Houston. Houston -- the nation's fourth
largest and most heavily polluted city -- has no zoning laws.

Quattrochi professes an ulterior motive for his work. He is concerned
with the prediction that by 2025, 80 percent of the world's population
will live in cities.

"We need to make cities more sustainable, habitable environments
-- for our kids, their kids and future progeny."

The Global Hydrology and Climate Center is a joint venture
between government and academia to study the global water cycle
and its effect on Earth's climate. Jointly funded by NASA and
its academic partners, and jointly operated by NASA's Marshall
Space Flight Center in Huntsville, Ala., and the University of
Alabama in Huntsville, the Center conducts research in a number
of critical areas.